The present invention relates to an oxygen generator, and more particularly, to a device for separating oxygen from a first gas and generating therefrom a second gas having a relatively high oxygen partial pressure.
Oxygen tends to move from a gas containing a high concentration of oxygen to one of lower concentration. If the two gases are separated from each other by an oxygen ion conductor, oxygen molecules will dissociate on one surface of the conductor and absorb electrons to form oxygen ions. These oxygen ions can then diffuse through the ionic conductor, leaving the entry surface with a deficiency of electrons. Emerging on the exit or low oxygen concentration side of the ion conductor, oxygen ions give up electrons to form molecular oxygen, leaving the exit surface with an excess of electrons. Thus, an electrical potential difference, or EMF, is set up between the two surfaces of the ion conductor. The greater the difference in oxygen content of the two gases, the greater will be the tendency of oxygen to diffuse through the conductor, and the greater will be potential difference between the entry and exit surfaces.
The EMF generated by the difference in oxygen partial pressures may be calculated by the Nernst relation: EQU EMF=t.sub.i (RT/nF) 1n (P.sub.02 /P'.sub.02). (Eq. 1)
where t.sub.i is the ionic transference number, R is the gas constant, T is the absolute temperature, n is the number of electrons involved in the electrode reaction, F is the Farady constant, and P.sub.02 and P'.sub.02 are the oxygen partial pressures in the first and second gases, respectively. In the present case, the electrode reaction is O.sub.2 +4e 20.sup.-2, and thus n=4.
These basic principles underly the operation of oxygen sensing devices, generally well known in the art. Oxygen sensors function by monitoring the EMF developed across an oxygen ion conductor which is exposed to gases having different oxygen partial pressures P.sub.02 and P'.sub.02.
The reciprocal principle underlies the operation of oxygen separators such as disclosed by Lawless U.S. Pat. No. 4,296,608. That is, where a voltage is applied to an oxygen ion conducting material, and if P.sub.02 =P'.sub.02, oxygen ions will be forced to flow across the material such that P.sub.02 =P'.sub.02. Thus, one gas will become richer in oxygen than the other, resulting in a basic oxygen separator.
As an alternative way of viewing the operation of an oxygen separator, consider an oxygen sensor in which a certain voltage signal V is generated by two gases in which P.sub.02 =P'.sub.02. Now, if a reverse voltage -V is applied to the material, the flow of oxygen ions through the material may be completely stopped. Increasing the magnitude of the negative voltage will then cause oxygen ions to flow in a reverse direction.
Consequently, an oxygen separator can be formed by operating an oxygen sensor in reverse. The above-mentioned Lawless patent provides a basic physical structure wherein these principles may be practically applied to separate oxygen from one gas stream to another.
A further need exists, however, to embody such basic structures in devices which may be practically operated to generate oxygen. What is needed, therefore, is a physical structure for such a separator that can provide a practical application of these principles.